For a wide range of timing applications; quartz crystals are used to supply a stable frequency reference with a typical accuracy of below +/−50 ppm over a typical temperature range of about 100° C. Improved accuracy, such as to approximately +/−1 ppm, can be achieved by frequency trimming of a combined quartz crystal resonator and oscillator circuitry system.
In some applications it is desirable to replace a quartz crystal with a micro-electromechanical system (MEMS) resonator. For example, efforts have recently been made to introduce radio frequency (RF) MEMS devices for timing applications. Compared with quartz crystals, MEMS resonators can provide reduced size as well as improved integration with an oscillator or application-specific integrated circuit (ASIC), thereby also providing reduced overall system costs.
To meet applications specifications, a MEMS resonator device often needs to have several characteristics at the same time. These characteristics can include high frequency stability, high Q-value, low supply voltage, low impedance supporting low power consumption, low phase noise and fast start-up behavior of combined system resonator-oscillator circuitry. To achieve high compatibility for different applications, it is desired to have a variable resonator frequency that is scalable by design rather than by process change. Additionally, a frequency trimming feature, such as by capacitive pulling or bias voltage trimming, is desired even for small ranges to enable a similar trimming procedure used for high accuracy quartz crystal applications. The performance parameters of resonator devices, however, depend on the process concept, such as materials, process stability and to a large extent on the resonator device design itself.